Unusual Attractive Au–π Interactions in Small Diacetylene‐Modified Gold Clusters

A golden π‐crust: Unprecedented attractive Au–π interactions were found between a σ‐coordinating C≡C moiety and neighboring gold atoms of diacetylene‐modified Au₈ clusters. The interaction was observed specifically for the diacetylenic ligands, and not found in the mono‐acetylenic analogues, indicating the uniqueness of the diacetylenic ligands.

Abstract

It is well known that alkynes act as π‐acids in the formation of complexes with metals. We found unprecedented attractive Au–π interactions in diacetylene‐modified [core+exo]‐type [Au₈]⁴⁺ clusters. The 4‐phenyl‐1,3‐butadiynyl‐modified cluster has unusually short Au–C_α distances in the crystal structure, revealing the presence of attractive interactions between the coordinating C≡C moieties and the neighboring bitetrahedral Au₆ core, which is further supported by IR and NMR spectra. Such weak interactions are not found in mono‐acetylene‐modified clusters, which indicates that they are specific for diacetylenic ligands. The attractive Au–π interactions are likely associated with the low energy of the π* orbital in the diacetylenic moieties, into which the valence electrons of the gold core may be back donated. The [Au₈]⁴⁺ clusters show clear red‐shifts of >10 nm with respect to the corresponding mono‐acetylenic clusters in UV/Vis absorption bands, which indicates substantial electronic perturbation effects of the Au–π interactions.

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